Cable Television (CATV) networks have been undergoing rapid upgrading, particularly in North America, in order to provide advanced services, such as digital video, telephony and Internet services. A major part of the upgrade cycle has been the development of the return path network—the path from residential subscribers to the CATV Headend. Previously, this portion of the network was used little, generally only for minor maintenance and billing operations. As interactivity needs grew and CATV technology advanced, this portion of the network became more significant. Now, the return path forms a critical portion of the system for two-way interactivity, such as telephony and data traffic.
The performance of digital return links can be compared favorably to their analog counterparts. Traditionally, the performance is flexibly traded off against bandwidth by simply truncating the Analog-to-Digital (A/D) word size to the desired amount of resolution. Smaller parallel word sizes translate to less serial bits-per-second to transmit, and thus lower line rates and lower required transmission bandwidths. Lower performance occurs by noting that the number of A/D converter bits sets the signal-to-noise ratio (SNR) for the signal being transported. When the signal is uniformly quantized, the classic relationship is 6 decibels (dB) per bit, meaning more bits translate into a higher SNR.
Because of the trade-off between performance and word size—and thus line signaling rate and transmission bandwidth—it is advantageous to find ways to improve the SNR from A/D conversion, but using lower resolution conversions.
The present invention is therefore directed to the problem of developing a method and apparatus for improving the SNR from an analog-to-digital conversion while using fewer bits of resolution. This SNR improvement can also be realized as an improvement in noise power ratio (NPR).